The present invention relates to xerographic copying machines. More particularly, the invention relates to a xerographic copying machine in which an electrostatic latent image or a toner image formed on the surface of an electrophotographic photosensitive element is transferred to an image transfer medium.
In an electrostatic latent image transfer type or toner image transfer type xerographic copying machine called "a PCC (Plain Paper Copier)", in general a photosensitive element in the form of a drum or in the form of an endless belt is employed with exposure of the photosensitive element being carried out by a scanning optical system in which a light beam in the form of a line or a spot is scanned in synchronization with the rotation of the photosensitive element. In a copying machine of this type, the size of the image in the direction of width (or in the axial direction) on the drum or the endless belt is determined from the magnification factor of the image projecting optical system while the size of the image in the direction of rotation of the drum or the endless belt (in a direction perpendicular to the axial direction) is determined from the speed of rotation of the drum or the endless belt or from the speed of movement of the optical image in the axial direction. Accordingly, in order to vary the magnification factor of the image in a xerographic copying machine using the above-described image scanning optical system, it is necessary to move the image scanning optical system or it is necessary to replace the image projecting optical system and to correspondingly change the speed of rotation of the photosensitive element or, if necessary, the speed of movement of the optical image, in synchronization with the operation of the optical system. Accordingly, such a copying machine is disadvantageous in that its mechanism is intricate. Furthermore, as a practical matter, in such a copying machine, the magnification factor can be varied only in two steps. Thus, the use of a xerographic copying machine using the above-described scanning optical system is not suitable for applications where it is necessary to frequently change the copying magnification factor as in a micropicture reader printer, a photographic enlarger and a process camera, or where it is necessary to steplessly change the copying magnification factor as in the case of a zooming operation.
On the other hand, in some electronic photograph copying machines called "CPC's (Coated Paper Copiers)" in which a toner image is formed directly on a xerographic photosensitive sheet such as a ZnO photosensitive sheet, exposure is carried out by a stationary optical system with the xerographic photosensitive sheet fixedly set horizontally in the exposure section in order to simplify the exposure operation as well as to permit variation of the copying magnification factor. In such a copying machine, in order to set the photosensitive sheet at least in the exposure section, it is necessary to intermittently drive the photosensitive sheet and to uniformly hold the photosensitive sheet over the entire exposure surface. Thus, this copying machine is disadvantageous in that it is unavoidably bulky. In addition, such a copying machine is clearly disadvantageous in that images cannot be copied on ordinary paper. In the case where such a technique is utilized for a PPC, the problem that images cannot be copied on ordinary paper may be solved but the other problems still remain.
Accordingly, an object of the invention is to provide a xerographic copying machine in which all of the above-described difficulties accompanying a conventional machine have been eliminated and electrostatic latent images or toner images can be transferred even in applications in which the copying magnification factor is frequently varied.